1. Field of the Invention
This invention relates to a method of stabilizing soil in pile bearing regions, especially in such regions which have liquefaction potential, and to a load supporting structure formed in such regions.
2. Description of the Prior Art
It is known that regions of loose, water-bearing, granular soils can liquify to depths of the order of fifty feet if subjected to seismic shock, such as is caused by earthquake or severe artificial earth vibration, and that in the event of such an occurrance, load bearing piles positioned in such regions can lose the support of the soil, both in bearing capacity and laterally. While the potential loss of bearing capacity in such regions is not too serious because piles can be driven to a depth below such regions to assure the required capacity, the potential loss of lateral support of the soil can be quite serious, indeed catastrophic. Thus, serious seismic disturbances involve both vertical and horizontal ground movement, the latter tending to cause lateral movement of a supported structure. Such movement, coupled with the structure's momentum, can induce very high lateral loads on the piles. Moreover, if liquefaction occurs, the passive soil pressures that help resist lateral displacement of the piles under lateral loads would greatly decrease thus causing extremely high bending moments in the piles so that very heavy or stiff piles would be required to resist such forces; and, upon loss of lateral soil support, the piles may tend to buckle under axial loads.
Another possible difficulty is the loss of frictional capacity of the upper portions of the piles due to loss of shear strength of the soil upon liquefaction of an upper soil stratum. Such a condition would transfer the structural loads to the lower portions of the piles with greater loads transferred to the lower bearing soil. This may result in overstressing of the piles, the lower soil, or both and may cause settlement of friction type piles.
In the construction of heavy structures in areas of soils having substantial water content, especially as it affects the construction of facilities such as power plants, the general procedure is to install a temporary well-point system so as to dewater the site and permit excavation in the dry. After completion of construction, the well-point system is removed and no further attempt is made to control ground water level. In one case of which I am aware, the ground water control system involved dewatering and water injection means to lower or raise the ground water level and was installed as a permanent system so as to control and regulate hydrostatic pressure beneath a foundation slab underlying the structure so as to maintain the load structure at a constant level. This procedure is expensive and time consuming and may still allow for liquefaction upon the occurrence of seismic shock.